This invention relates generally to an electronic halftoning system for reproducing images and more specifically to such a system wherein detail contrast and large area contrast are controlled independently of each other.
Halftoning, also frequently referred to as "screening", generally comprises reproduction of an image in which the gray scale values of a continuous tone image are to be represented using only varying areas of either black or white. Various techniques for electronic halftoning have been disclosed. In electronic halftoning a signal representing the image information is combined with the halftone screen and thresholded electronically so that the output electronic signal is binary. This tends to reduce the reliance of the system on controlled thresholding of the image recording process; however, the threshold level still has some effect.
According to a preferred prior art electronic halftoning technique, a simulation of the photographic process is provided by electronic means. Each electronic halftone dot is generated from a large number of subcells, each of which is individually turned on or off. In order to provide a high quality halftone a separate sample (also referred to as a "pixel") of the image to be reproduced is utilized in making a decision as to whether to turn on or off each subdot element. In this method there is combined (typically by addition) a halftone function (i.e., a periodic function unrelated to image detail) with the electronic signal corresponding to the image information. This combined signal is then compared with a fixed threshold to determine whether to turn the spot on or off. Typically, levels above threshold are made white in the reproduction and levels below threshold are made black, although this is arbitrary and the reverse may be true. Hence, the continuous tone original image becomes a binary image suitable for printing. In a digital implementation, signals for the screen and picture functions are sampled. Typically, there may be sixteen or more samples within the area corresponding to one period of the two dimensional screen function. Dots of various size represent the gray levels. Dots can also change shape or position as well as size and thus tend to represent image detail finer than the halftone screen period. This method has been described, for example, by Klensch, R. J., "Electronically Generated Halftone Pictures", RCA Review, September, 1970 and Bayer, B. E., "An Optimum Methof for Two-Level Rendition of Continuous-Tone Pictures", IEEE International Conference on Communications, Vol. 1, 1973.
The prior art electronic halftoning techniques have not been found to be entirely satisfactory. For example, if the screening process is adjusted to give good tone reproduction over the full range of grays, the systems typically become poor for reproducing text unless the text is of very high contrast. Also, if the original image includes very fine detail the systems typically produce errors in gray scale, tending to fail to reproduce the gray scale properly and the closer the detail of the image resembles the screen, such as where the original image is itself a halftone image, the more severe the failure becomes, typically resulting in the occurrence of undesirable Moire patterns in the unscreened image. Another deficiency of prior art electronic halftoning methods is that no possibility exists for introducing simultaneous sharpening of the image.